The present invention relates to a new and improved construction for an electrostatic powder coating installation. The coating of articles or the like by means of electrostatically charged powder is widely used in fabrication or manufacturing operations. In a typical electrostatic powder coating operation, the powder is charged at the nozzle of a spraying device to an extremely high electrical potential, and the article to be coated is electrically grounded whereby the discharged powder is attracted to the exposed surfaces of the article. However, even under ideal conditions a portion of the electrostatically charged powder does not impinge upon the article to be coated. Rather, the excess powder or overspray remains in the atmosphere surrounding the coated article. Consequently, the prior art has offered several proposals for means to remove the overspray from the surrounding atmosphere and to recover the powder.
One advantageous example of such a prior art powder recovery installation is found in the Lehmann U.S. Pat. No. 3,918,641, issued Nov. 11, 1975 and assigned to Gema, A. G. Apparatebau of St. Gallen, Switzerland. In accordance with the teachings of the Lehmann patent, electrostatic powder spraying means and the article to be coated are enclosed within a spray booth. The floor of the booth is formed by one face of a filter moving through the booth. The other face of the movable filter confronts a suction device effective through the filter to maintain the interior of the booth at a slightly negative pressure. Thus, almost all of the powder overspray will be attracted to and deposited on the exposed face of the movable filter.
A cleaning device is operatively associated with the filter in order to remove the deposited powder therefrom. The cleaning device comprises a suction nozzle directed towards the filter and connected with the inlet of a suction blower. The outlet of the suction blower is connected, via an air-powder separator, with the supply container of the spray nozzle, enabling the recovered powder overspray to be recirculated back into the spraying system. The above described prior art proposal therefore provides an extremely advantageous and practical apparatus for the removal and recirculation of the powder overspray.
It is a primary objective of the present invention to improve powder recovery installations of the general type disclosed in the aforementioned Lehmann patent by providing means to greatly increase the useable life of a movable filter in such installations. In its broadest scope the invention provides a movable filter assembly comprising a first filter layer which faces the spray booth interior. The filter layer is supported on and moved through the spray booth by a second layer, which may advantageously be of a metallic mesh construction to provide a rugged yet, flexible filter construction. Means are provided adjacent the cleaning device to separate the support layer from the filter layer whereby friction between the filter layer and the cleaning device is greatly reduced to extend the useable life of the movable filter.
In one advantageous form of the invention, the filter layer and support layer are together in the form of an endless belt which is diverted around two spaced rollers. The endless belt is arranged such that the upper run thereof passes through and forms a portion of the floor of the spray booth. The overspray is deposited on the filter layer. The endless belt passes then through a cleaning station which includes a suction nozzle arranged to contact the filter band. A diverting roller is positioned directly below the suction nozzle whereby the support band is diverted around the diverting roller and away from the suction nozzle while the filter band is caused to pass in contact with the suction nozzle.
In any of the forms of the present invention, the vacuum effect of the suction nozzle is fully effective to remove the overspray deposited on the filter band. However, the friction between the filter band and the suction nozzle will be greatly reduced inasmuch as the heavier support layer does not push up against the filter band to press it against the suction nozzle. This will minimize wear and tear on the filter and extend its useable life.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description, and to the accompanying drawings.